1. Field of the Invention
The subject matter of this invention relates generally to motor startup protection circuits and relates specifically to those circuits which utilize apparatus for preventing the tripping of a motor protecting circuit breaker during the acceleration of the motor after startup.
2. Description of the Prior Art
High inertia electrical motors often require a substantial period of time to reach normal operating speed after startup has been initated. During this period relatively high line currents are often drawn. It is generally well known that these currents are not excessively dangerous and they generally persist no longer that the time it takes for the motor to reach normal running speed. However, there are other instances when line currents may be drawn which are not due to startup but which may be due to faults or the like. It is desirous in the latter situation to sense excessively high line currents and to provide a signal to an inverse time overload logic module to trip a circuit breaker to protect the motor and the associated lines. Generally the inverse time overload logic module provides an output signal which is inversely related to the value of the line current. This means if the line current is only slightly above rated value it will take a long time after it is initially sensed to trip the circuit breaker. However, if the line current is substantially higher, it will take a shorter time before the circuit breaker is tripped. In the past it has been taught to utilize a current sensor which senses overload current in the line to be protected and provides an output voltage which is related to the overload current. The inverse time overload logic module reacts, as was described previuosly, as a functon of the output voltage. In the past the utilization of a long acceleration module disposed in parallel with the voltage input has been taught. This long acceleration module comprised a Zener diode which was connected across the inverse time overload logic module input to limit the voltage which the current sensor could provide to the latter mentioned module regardless of the amount of current flowing in the lines. The theory behind this was that the highest current likely to occur on the line other than fault current was usually drawn during the acceleration of the motor after startup. The long acceleration module prevented a voltage which would normally be related to this current from appearing across the inverse time overload logic input terminal. However, this presented a disadvantage. If exceptionally high currents occured because of a fault or similar phenomena on the lines after motor startup, the circuit breaker could not be tripped as quickly as might be desired because the long acceleration module would prevent the inverse time overload logic module from providing an output signal at this time to its circuit breaker. It would therefore be advantageous if a long acceleration module could be utilized in conjunction with an inverse time overload logic module during motor startup so that the effects of motor startup on the line currents could be ignored by the inverse time overload logic module. However, after startup had been completed the long acceleration module could be effectively disconnected from the inverse time overload logic module so that any subsequent high current could be interrupted if necessary with high speed by the inverse time overload logic module and its circuit breaker.